Carbonylation process using metal-polydentate ligand catalysts

ABSTRACT

A process for the liquid phase carbonylation of an alcohol and/or a reactive derivative thereof in the presence of hydrogen in which there is employed a catalyst comprising rhodium of iridium coordinated with a polydentate ligand.

This application is the U.S. National Phase of International ApplicationPCT/GB04/001900 filed 5 May 2004 which designated the U.S.PCT/GB04/001900 claims priority to British Application No. 0311092.1filed 14 May 2003. The entire content of these applications areincorporated herein by reference.

The present invention relates in general to a process for the liquidphase carbonylation of an alcohol and/or a reactive derivative thereof.In particular the present invention relates to the liquid phasecarbonylation of an alcohol and/or a reactive derivative thereof in thepresence of hydrogen and a catalyst comprising rhodium or iridiumcoordinated with a polydentate ligand.

Preparation of carboxylic acids by rhodium-catalysed carbonylationprocesses is known and is described, for example, in EP-A-0632006 andU.S. Pat. No. 4,670,570.

EP-A-0632006 discloses a process for the liquid phase carbonylation ofmethanol or a reactive derivative thereof which process comprisescontacting carbon monoxide with a liquid reaction composition comprisingmethanol or a reactive derivative thereof, a halogen promoter and arhodium catalyst system comprising a rhodium component and a bidentatephosphorus-sulphur ligand, the ligand comprising a phosphorus dativecentre linked to a sulphur dative or anionic centre by a substantiallyunreactive backbone structure comprising two connecting carbon atoms ora connecting carbon and a connecting phosphorus atom.

Preparation of carboxylic acids by iridium-catalysed carbonylationprocesses is known and is described, for example in EP-A-0786447,EP-A0643034 and EP-A-0752406.

EP-A-0643034 describes a process for the production of acetic acid bycarbonylation of methanol or a reactive derivative thereof which processcomprises contacting methanol or a reactive derivative thereof withcarbon monoxide in a liquid reaction composition in a carbonylationreactor characterised in that the liquid composition comprises (a)acetic acid, (b) an iridium catalyst, (c) methyl iodide, (d) at least afinite quantity of water, (e) methyl acetate and (f) as promoter, atleast one of ruthenium and osmium.

The use of bidentate chelating phosphorus or arsenic ligands incarbonylation processes is known, for example, from GB 2,336,154, U.S.Pat. No. 4,102,920 and U.S. Pat. No. 4,102,921.

GB 2,336,154 describes a process for the liquid-phase carbonylation ofan alcohol and/or a reactive derivative thereof to produce a carboxylicacid in the presence of a bidentate ligand of formula R¹R²X—Z—YR⁵R⁶,wherein X and Y are independently N, P, As, Sb or Bi, and Z is adivalent linking group.

U.S. Pat. No. 4,102,920 describes a process for the carbonylation ofalcohols, esters, ethers and organo halides in the presence of a rhodiumcomplex with a polydentate phosphine or arsenic chelating ligand. U.S.Pat. No. 4,102,921 describes a similar process in the presence of aniridium complex with a polydentate phosphine or arsenic chelatingligand.

The carbonylation of methanol to produce acetic acid, the presence ofhydrogen is known to result in the formation of undesirable liquidby-products such as acetaldehyde, ethanol and propionic acid. Propionicacid requires an expensive and energy intensive distillation column toseparate it from the acetic acid product. Furthermore acetaldehyde canundergo a series of condensation and other reactions to yield,eventually, higher organic iodide compounds. Some of these materials,especially, for example, hexyl iodide, are difficult to remove byconventional distillation and further treatment steps are sometimesnecessary to give acetic acid of sufficient purity. EP-A-0 849 251,which describes an iridium catalysed process for the carbonylation ofmethanol to acetic acid, states that the amount of hydrogen in thecarbon monoxide feed is preferably less than 1 mol % and the hydrogenpartial pressure in the reactor is preferably less than 1 bar.Similarly, EP-A-0 728 727, which describes a rhodium catalysed processfor the carbonylation of methanol to acetic acid, states that thehydrogen partial pressure in the reactor is preferably less than 2 bar.

It has also been found that, using certain rhodium catalysts formethanol carbonylation, the presence of hydrogen in the carbon monoxidefeed leads to the production of ethanol and acetaldehyde with only minoramounts of acetic acid being produced.

U.S. Pat. No. 4,727,200, for example, describes a process for thehomologation of an alcohol by reaction with synthesis gas using arhodium-containing catalyst system. The major product formed with asynthesis gas feed is ethanol, acetic acid being a relatively minorby-product.

Moloy et al. (Organometallics, 1989, 8, pp 2883-2893) describe a processfor the rhodium-catalysed reductive carbonylation of methanol utilisingsynthesis gas in the presence of a diphosphine ligand to produce highlevels of acetaldehyde. Addition of ruthenium to the catalyst favourshydrogenation to produce ethanol.

Thus, there remains a need for an improved process for the production ofcarboxylic acids and/or the alcohol esters of carboxylic acids by thecatalytic carbonylation of an alcohol and/or a reactive derivativethereof. In particular there remains a need for a carbonylation processwhich is tolerant towards the presence of hydrogen in that only smallquantities of or no liquid hydrogenation by-products are produced.

It has now been found that an improved carbonylation process may beachieved by employing a catalyst comprising rhodium or iridiumcoordinated with a polydentate ligand wherein said ligand has a biteangle of at least 145° or is coordinated to the rhodium or iridium metalin a rigid structural conformation. Advantageously, the catalystsemployed in the process of the present invention have been found to haveimproved tolerance of hydrogen present in the carbonylation process inthat no or small quantities of liquid by-products are formed in theprocess. In addition, the metal-polydentate ligand complexes accordingto the present invention may have higher stability in the carbonylationprocess than non-rigid metal-ligand complexes or complexes havingligands with a bite angle of less than 145°. Furthermore, the process ofthe present invention can be carried out in the absence of aconventional catalyst stabiliser compound such as lithium iodide.

Accordingly, the present invention provides a process for the productionof a carboxylic acid and/or the alcohol ester of a carboxylic acid,which process comprises carbonylating an alcohol and/or a reactivederivative thereof with carbon monoxide in a liquid reaction compositionin a carbonylation reactor, said liquid reaction composition comprisingsaid alcohol and/or a reactive derivative thereof, a carbonylationcatalyst and an alkyl halide co-catalyst and optionally a finiteconcentration of water, wherein said catalyst comprises at least one ofrhodium or iridium which is coordinated with a polydentate ligandwherein said polydentate ligand has a bite angle of at least 145° orforms a rigid Rh or Ir metal-ligand complex and wherein said polydentateligand comprises at least two coordinating groups which, independentlycontain P, N, As or Sb as the coordinating atom of at least two of theco-ordinating groups and wherein in said process there is maintainedhydrogen at a hydrogen:CO mole ratio of at least 1:100 and/or the carbonmonoxide feed to the carbonylation reactor contains at least 1 mol %hydrogen.

The polydentate ligand comprises at least two coordinating groups which,independently, contain P, N, As or Sb as the coordinating atom (donoratom) in at least two of the co-ordinating groups. The two coordinatinggroups may be represented, respectively, as L1 and L2.

The polydentate ligand, when complexed with the rhodium or iridium metalcentre (atom), will form a ring structure comprising the metal atom, thecoordinating P, N, As or Sb atoms and the ligand backbone. “Rigidmetal-ligand complex”, as used herein, means that the ring structure hasa rigid conformation. The degree of rigidity of a metal-ligand complexmay be derived by the skilled man based on the structure of the ligandand its expected bonding configuration. Rigidity may be defined ingeneral terms by consideration of the structure of the ligand-metalcomplex formed, or, for a more accurate definition, may be definedmathematically, for example in terms of the “flexibility range” of theligand. “Flexibility range” as used herein, is defined as the range ofbite angles accessible for the L1-M-L2 angle (wherein the L1-M-L2 angleis the angle formed by the two co-ordinating groups and the metalcentre, M, wherein M is Rh or Ir), for example, within 3 kcal/mol of theminimum energy. The bite angle and flexibility range for a bidentateligand may be derived from the potential energy diagram calculatedaccording to the method of Casey et al. in Israel Journal of Chemistry,Vol. 30 (1990), p. 299-304, the contents of which are hereinincorporated by reference. Preferably, for the catalysts of the presentinvention, the flexibility range is less than 40°, preferably less than30°. Similar calculations may be used to define the flexibility rangefor non-bidentate ligands.

Preferably, the co-ordinating groups, L1 and L2 each contain phosphorusas the coordinating atom. Such phosphorus-containing groups, referred tohereinafter as P1 and P2, preferably have general formula R¹R²P andR³R⁴P, wherein R¹, R², R³ and R⁴ are each independently selected fromunsubstituted or substituted alkenyl groups, alkyl groups and arylgroups, especially phenyl groups. Preferably R¹, R², R³ and R⁴ are eacha phenyl group. One or more of the phenyl groups may be substituted orunsubstituted. For example, each of P1 and P2 may be a diphenylphosphinegroup (PPh₂). Alternatively, one or more of the R¹, R², R³ and R⁴ phenylgroups in the P1 and P2 groups may be substituted. Suitably, the phenylgroups may be substituted at one or more of the ortho positions by atleast one group selected from alkyl, aryl and alkyloxy (OR) groups.Particularly preferred ortho-substituents are Me, CF₃, Et, iso-Pr andOMe.

To improve the solubility of the polydentate ligand and hence thecatalyst in the liquid reaction composition one or more of the R¹, R²,R³, and R⁴ groups on the co-ordinating groups may be substituted withone or more hydrophilic and/or polar groups. Examples of such groupsinclude —CO₂H, —CO₂Me, —OH, —SO₃H, —SO₃Na, —NH₂, —NH₃ ⁺ and —NR₂H⁺.

The rigid conformation of a polydentate metal-ligand complex will be thedirect result of the ligand structure. In particular, where thepolydentate ligand is a bidentate ligand, the ligand should havehindered rotation along the ligand backbone. The ligand backbone, asdefined herein, is the part or parts of the ligand which will form thering structure (comprising the metal atom and the coordinating (donor)atoms) in the metal-ligand complex. For example, the rigid conformationmay be the result of a vinylic or an aromatic backbone between thecoordinating groups L1 and L2, which hinders or prevents rotation of theligand backbone. Alternatively, or additionally, the ligand-metalcomplex may be rigid due to steric effects that hinder rotation of theligand backbone.

Suitable rigid bidentate phosphine-containing ligands include those ofgeneral structures 1 to 3 below; wherein P1 and P2 are R¹R²P and R³R⁴Prespectively and wherein R¹—R⁴ are as defined above:

Each of structures (1 to 3) will form metal-bidentate ligand complexeswith a rigid conformation. For example, ligands of general structure 1will form five-membered rings with the metal centre, the structures ofwhich are rigid due to the vinylic backbone. R₅ and R₆ in structure 1may, independently, be selected from H, alkyl and aryl. R₅ and R₆ may belinked to form an aromatic ring, for example a phenyl ring, as shown instructure 1a below.

Ligands of general structures 2 and 3 will form rigid six and sevenmembered rings respectively. In particular, it is believed that rotationof the ligand of structure 3 about the single bridge bond is preventedby the steric hindrance of overlapping hydrogen atoms in the structure.

Suitably, the structures 1, 1a, 2 and 3 above may be substituted by oneor more substituents, such as by one or more alkyl groups, includingsubstitution of the P1 and/or P2 groups.

In particular, R¹, R², R³ and R⁴ of the P1 and P2 groups present instructures 1, 1a, 2 and 3 above are preferably each independentlyselected from phenyl groups and substituted phenyl groups. Morepreferably one or more of the R¹, R², R³ and R⁴ groups are substituted,preferably at one or more of the ortho positions. Preferredortho-substituents include alkyl, aryl or alkyloxy (OR) groups.Particularly preferred ortho-substituents are Me, CF₃, Et, iso-Pr andOMe.

To improve the solubility of the bidentate ligands represented by thestructures 1, 1a, 2 and 3 above, and thus the catalyst in the liquidreaction composition, the bidentate ligands may be substituted with oneor more hydrophilic and/or polar groups. Preferably one or more of thephosphorous-containing groups of the bidentate ligand is substituted.Examples of suitable substituents include —CO₂H, —CO₂Me, —OH, —SO₃H,—SO₃Na, —NH₂, —NH₃ ⁺ and —NR₂H⁺.

Preferred bidentate arsine and stibine ligands may be represented bystructures 1, 1a, 2 and 3 above, or variants thereof as described, andwherein the phosphorus atoms are replaced by arsenic or antimony atoms.Preferred mixed bidentate ligands include structures 1, 1a, 2 and 3above, or variants thereof as described, and which comprise acombination of two groups selected from phosphorus, arsenic andantimony-containing groups.

Preferred bidentate nitrogen-containing ligands are aromatic ringsystems which contain nitrogen as the donor atom. The aromatic rings maybe either substituted or unsubstituted and the ring system may alsocomprise other heteroatoms such as oxygen. Examples of suitable ringsystems include substituted and unsubstituted bipyridines.

The polydentate ligand of the present invention may also be a tridentateligand.

The tridentate ligand has three coordinating groups through which theligand coordinates to the rhodium or iridium metal centre. The threecoordinating groups may be represented by L1 and L2, as definedpreviously, and L3, a third coordinating group, which preferablycontains P, As, Sb, O, N, S or carbene as the donor (co-ordinating)atom.

Preferably the tridentate ligand is represented by the formulaL1(R⁷)L3(R⁸)L2, wherein R⁷ and R⁸ are linking groups that link L1 to L3and L3 to L2 respectively. The linking groups R⁷ and R⁸ areindependently selected from aryl and alkenyl groups, preferably vinylicor phenyl groups. R⁷ and R⁸ may themselves form at least one cyclicstructure comprising L3, which may be represented by the genericstructure A below:

Preferably, the tridentate ligand is represented by the formulaL1(R⁷)L3(R⁸)L2 as described above, and coordinates to the rhodium oriridium catalyst metal centre in a bridging conformation, such that L1and L2 are mutually trans with respect to the metal centre. By mutuallytrans, as used throughout this specification, is meant that the angleformed by the two ligands and the metal centre, L1-M-L2, wherein M isthe Rh or Ir metal centre, is at least 145°, preferably at least 150°.The angle may be measured using conventional techniques, such as X-raycrystallography.

Preferably, the tridentate ligand co-ordinates such that the donor atomsin the L1, L2 and L3 groups are in a meridional (mer-) co-ordinationmode with respect to the metal centre. More preferably, the tridentateligand co-ordinates such that the donor atoms of the L1, L2 and L3groups are in an essentially planar configuration with respect to themetal centre.

Preferably, L1 and L2 are phosphorous-containing groups and L3 isoxygen, such that the tridentate ligand has the formula P1-R⁷—O—R⁸-P2,wherein P1 and P2 are phosphine-containing groups of general formulaR¹R²P and R³R⁴P, and wherein R¹, R², R³ and R⁴ are each independentlyselected from unsubstituted or substituted alkenyl groups, alkyl groups,aryl groups, especially phenyl groups. Preferably, R¹, R², R³ and R⁴ inthe tridentate ligand are each a phenyl group. Each of the phenyl groupsmay be substituted or unsubstituted. Each of P1 and P2 may bediphenylphosphine (PPh₂). Alternatively, one or more of the R¹, R², R³and R⁴ phenyl groups in the P1 and P2 groups are substituted. Suitablythe phenyl groups may be substituted at one or more of the orthopositions by at least one group selected from alkyl, aryl or alkyloxy(OR) groups. Particularly preferred ortho substituents are Me, CF₃, Et,iso-Pr and OMe.

To improve the solubility of the tridentate ligand, and hence thecatalyst, in the liquid reaction composition one or more of the R¹, R²,R³, R⁴, R⁷ and R⁸ groups on the tridentate ligand may be substitutedwith one or more hydrophilic and/or polar groups. Examples of suitablesubstituents include —CO₂H, —CO₂Me, —OH, —SO₃H, —SO₃Na, —NH₂, —NH₃ ⁺ and—NR₂H⁺.

The rigid conformation of the tridentate metal-ligand complex may be thedirect result of the ligand structure or may be a result of thestructure of the metal ligand complex. For example, the rigidconformation may be the result of a rigid structure of the overallligand, such as Xantphos (structure 4 below). Thus, the tridentateXantphos ligand, when complexed with the rhodium or iridium metal centre(atom), forms a rigid ring structure comprising the metal atom, thecoordinating P, As or Sb atoms and the ligand backbone (having oxygen asthe third donor).

Alternatively, the rigid conformation may be the result of R⁷ and R⁸each being, independently, a vinylic or an aromatic backbone, whichhinder or prevent rotation of the ligand backbone between L1 and L3, andbetween L3 and L2 respectively, but where the overall ligand is rigidonly when coordinated to a metal centre;. An example of such a structureis DPEphos, which is shown as structure 5 below. In this example, theligand, when coordinated to the rhodium or iridium metal centre, forms arigid ring structure comprising two rigid five-membered rings that givean overall rigidity to the ligand-metal complex. Alternatively, oradditionally, the ligand-metal complex may be rigid due to stericeffects that hinder rotation of the ligand backbone, as describedpreviously for structure 3.

Specific examples of suitable tridentate phosphine-containing ligandsfor use in the present invention include Xantphos, Thixantphos,Sixantphos, Homoxantphos, Phosxantphos, Isopropxantphos, Nixantphos,Benzoxantphos, DPEphos, DBFphos and R-Nixantphos, having structures 4-14which are given below. The R grouping of R-Nixantphos is preferablyselected from alkyl and aryl groups, and more preferably selected frommethyl, ethyl, propyl and benzyl.

Suitably, structures 4 to 14 above, may be substituted by one or moresubstituents, such as one or more alkyl groups, for example. Thestructure of t-Bu-xantphos is shown as structure 15 below.

In the tridentate phosphine-containing ligands represented by structures4 to 15, the diphenylphosphine groups may be replaced by P1 and P2groups as previously defied. In particular, preferred P1 and P2 groupsare R¹R²P and R³R⁴P groups wherein R¹, R², R³ and R⁴ are each,independently selected from phenyl groups and substituted phenyl groupsand one or more of the R¹, R², R³ and R⁴ groups are substituted,preferably at one or more of the ortho positions, with alkyl, aryl oralkyloxy (OR) groups. Particularly preferred ortho-substituents are Me,CF₃, Et, iso-Pr and OMe.

To improve the solubility of the tridentate ligands represented bystructures 4 to 15, and thus the catalyst, in the liquid reactioncomposition, the tridentate ligands may be substituted with one or morehydrophilic and/or polar groups, especially on one or more of thephosphine groups on the tridentate ligand. Examples of suitablesubstituents include —CO₂H, —CO₂Me, —OH, —SO₃H, —SO₃Na, —NH₂, —NH₃ ⁺ and—NR₂H⁺.

Suitably, the tridentate phosphine containing ligands of any of theabove structures 4 to 15, or substituted variants thereof as describedabove, may have the oxygen atom in L3 substituted by a sulphur atom or anitrogen atom.

Preferred tridentate arsine- and stibine-containing ligands includestructures 4 to 15 above, or variants thereof as described herein,wherein the phosphorus atoms are replaced by arsenic or antimony atoms.Preferred mixed tridentate ligands include structures 4 to 15 above, orvariants thereof as described herein, comprising, as L1 and L2, acombination of two groups selected from phosphorus, arsenic andantimony-containing groups.

For example, the structures of As, As-t-Bu-xantphos and P,As-t-Bu-xantphos are given below as structures 16 and 17 respectively.

Preferred tridentate nitrogen-containing ligands are aromatic ringsystems which contain nitrogen as the donor atom. The aromatic rings maybe either substituted or unsubstituted and the ring system may alsocomprise other heteroatoms such as oxygen. Examples of suitable ringsystems include substituted and unsubstituted terpyridines.

The bidentate and tridentate ligands are either commercially availableor may be synthesised according to methods known in the art. Morespecifically, the tridentate ligands represented by structures 4 to 17,and variants thereof as described, may be synthesised according tomethods as described or analogous to those described by van der Veen etal., Chem. Commun., 2000, 333, the contents of which are hereinincorporated by reference.

The use of a catalyst that comprises rhodium or iridium coordinated witha polydentate ligand in a rigid structural conformation or which has abite angle of at least 145° according to the present invention has beenfound to give improved selectivity to carboxylic acid products andreduced selectivity to liquid hydrogenation by-products, such asalcohols and aldehydes, in the presence of hydrogen.

Preferably, the catalyst of the present invention comprises rhodium. Theproposed mechanisms of rhodium catalysed carbonylation and reductivecarbonylation can be found, for example, in Moloy et al.,Organometallics, Vol. 8, No. 12, 1989, the contents of which are hereinincorporated by reference. Without wishing to be bound by theory it isbelieved that the rigid conformation of the metal-ligand complexesaccording to the present invention prevents or at least inhibits theability of the metal-ligand complex to change conformation, which inturn prevents or at least inhibits hydrogen addition to the metal-ligandcomplex or prevents the elimination of an aldehyde (e.g. acetaldehyde)from a metal acyl species (e.g. M-COCH₃) formed during carbonylation,such an elimination reaction requiring either H₂ to enter a vacant sitecis to the acyl group or a reductive elimination reaction between ametal hydride ligand (formed via H₂ addition) and metal acyl ligandwhich are mutually cis. For example, in the case of a metal complex witha square pyramid structure containing a rigid bidentate ligand with anapical acyl group (e.g. COMe) and two halide ligands (e.g. I) the vacantsite is fixed in a position trans to the acyl group, thereby preventingits reaction with hydrogen to form an aldehyde.

In addition, and again without wishing to be bound by theory, it is alsobelieved that the tridentate ligands, by coordinating with three donors,may have an additional steric blocking effect that prevents or inhibitshydrogen addition to the metal-ligand complex.

The catalyst of the present invention may be prepared by coordinating aniridium- or rhodium-containing compound with a polydentate ligand. Thecatalyst may be formed in situ in the liquid reaction composition, bythe separate addition of an iridium- or rhodium-containing compound anda polydentate ligand to the liquid reaction composition. The iridium- orrhodium-containing compound may be added in any suitable form whichdissolves in the liquid reaction composition or is convertible to asoluble form. Preferably, however, the catalyst is added to the liquidreaction composition in the form of a pre-formed metal-polydentateligand complex in which the polydentate ligand is coordinated to theiridium- or rhodium-containing compound. The pre-formedmetal-polydentate ligand complex may be prepared, for example, by mixinga suitable iridium- or rhodium-containing compound having displaceablegroups with the polydentate ligand in a suitable solvent, for examplemethanol, prior to addition to the liquid reaction composition.

Examples of pre-formed iridium-tridentate ligand complexes include[{L1(R⁷)L3(R⁸)L2}Ir(COMe)I₂], [{L1(R⁷)L3(R⁸)L2}Ir(CO)I],[{L1(R⁷)L3(R⁸)L2}Ir(CO)]⁺ and [{L1(R⁷)L3(R⁸)L2}IrI(CO)Me]⁺, whereinL1(R⁷)L3(R⁸)L2 represents the tridentate ligand as hereinbeforedescribed.

Examples of pre-formed rhodium-tridentate ligand complexes include[{L1(R⁷)L3(R⁸)L2}Rh(COMe)I₂], [{L1(R⁷)L3(R⁸)L}Rh(CO)I],[{L1(R⁷)L3(R⁸)L2}Rh(CO)]⁺ and [{L1(R⁷)L3(R⁸)L2}RhI(CO)Me]⁺, whereinL1(R⁷)L3(R⁸)L2 represents the tridentate ligand as previously described,for example [{Xantphos}Rh(COMe)I₂].

Preferably the iridium- or rhodium-containing compound is a chloridefree compound, such as an acetate, which is soluble in one or more ofthe liquid reaction composition components, and so may be added to thereaction as a solution therein.

Examples of suitable iridium-containing compounds include IrCl₃, IrI₃,IrBr₃, [Ir(CO)₂I]₂, [Ir(CO)₂Cl]₂, [Ir(CO)₂Br]₂, [Ir(CO)₄I₂]⁻H⁺,[Ir(CO)₂Br₂]⁻H⁺, [Ir(CO)₂I₂]⁻H⁺, [Ir(CH₃)I₃(CO)₂]⁻H⁺, Ir₄(CO)₁₂,IrCl₃.4H₂O, IrBr₃.4H₂O, Ir₃(CO)₁₂, iridium metal, Ir₂O₃, IrO₂,Ir(acac)(CO)₂, Ir(acac)₃, iridium acetate, [Ir₃O(OAc)₆(H₂O)₃][OAc], andhexachloroiridic acid H₂[IrCl₆], preferably, chloride-free complexes ofiridium such as acetates, oxalates and acetoacetates.

Examples of suitable rhodium-containing compounds include [Rh(CO)₂Cl]₂,[Rh(CO)₂I]₂, [Rh(Cod)Cl]₂, rhodium (III) chloride, rhodium (III)chloride trihydrate, rhodium (III) bromide, rhodium (III) iodide,rhodium (III) acetate, rhodium dicarbonylacetylacetonate, RhCl(PPh₃)₃and RhCl(CO)(PPh₃)₂.

Preferably, the concentration of iridium in the liquid reactioncomposition is in the range 100 to 6000 ppm by weight of iridium, morepreferably in the range 400 to 5000 ppm, such as in the range 500 to3000 ppm by weight.

Preferably, the concentration of rhodium in the liquid reactioncomposition is in the range 25 to 5000 ppm by weight of rhodium, morepreferably, in the range 250 to 3500 ppm.

The mole ratio of the rhodium or iridium metal to the polydentate ligandin the reactor is optimally approximately 1:1, especially where apre-formed metal-ligand complex is employed. Alternatively, an excess ofligand may be present in the liquid reaction composition, especially,for example, where the metal-ligand complex is to be formed in-situ.Thus, the mole ratio of the rhodium or iridium metal to the polydentateligand may be less than 1:1, suitably be in the range from 1:1 to 1:2.

The liquid reaction composition may also comprise a promoter metal.Suitable promoters are selected from ruthenium, osmium, rhenium,cadmium, mercury, zinc, gallium, indium and tungsten. Preferredpromoters are selected from ruthenium and osmium, most preferably,ruthenium. The promoter may comprise any suitable promotermetal-containing compound which is soluble in the liquid reactioncomposition. The promoter may be added to the liquid reactioncomposition for the carbonylation reaction in any suitable form whichdissolves in the liquid reaction composition or is convertible tosoluble form.

Examples of suitable ruthenium-containing compounds which may be used assources of promoter include ruthenium (III) chloride, ruthenium (III)chloride trihydrate, ruthenium (III) chloride, ruthenium (III) bromide,ruthenium metal, ruthenium oxides, ruthenium (III) formate,[Ru(CO)₃I₃]−H+, [Ru(CO)₂I₂]_(n), [Ru(CO)₄I₂], [Ru(CO)₃I₂]₂,tetra(aceto)chlororuthenium(II,III), ruthenium (III) acetate, ruthenium(III) propionate, ruthenium (III) butyrate, ruthenium pentacarbonyl,trirutheniumdodecacarbonyl and mixed ruthenium halocarbonyls such asdichlorotricarbonylruthenium (II) dimer, dibromotricarbonylruthenium(II) dimer, and other organoruthenium complexes such as tetrachlorobis(4-cymene)diruthenium(II), tetrachlorobis(benzene)diruthenium(II),dichloro(cycloocta-1,5diene) ruthenium (II) polymer andtris(acetylacetonate)ruthenium (III).

Examples of suitable osmium-containing compounds which may be used assources of promoter include osmium (III) chloride hydrate and anhydrous,osmium metal, osmium tetraoxide, triosmiumdodecacarbonyl, [Os(CO)₄I₂],[Os(CO)₃I₂]₂, [Os(CO)₃I₃]−H+, pentachloro-μ-nitrodiosmium and mixedosmium halocarbonyls such as tricarbonyldichloroosmium (II) dimer andother organoosmium complexes.

Examples of suitable rhenium-containing compounds which may be used assources of promoter include Re₂(CO)₁₀, Re(CO)₅Cl, Re(CO)₅Br, Re(CO)₅I,ReCl₃.xH₂O, [Re(CO)₄I]₂, Re(CO)₄I₂]⁻H⁺ and ReCl₅.yH₂O.

Examples of suitable cadmium-containing compounds which may be usedinclude Cd(OAc)₂, CdI₂, CdBr₂, CdCl₂, Cd(OH)₂, and cadmiumacetylacetonate.

Examples of suitable mercury-containing compounds which may be usedinclude Hg(OAc)₂, HgI₂, HgBr₂, HgCl₂, Hg₂I₂, and Hg₂Cl₂.

Examples of suitable zinc-containing compounds which may be used includeZn(OAc)₂, Zn(OH)₂, ZnI₂, ZnBr₂, ZnCl₂, and zinc acetylacetonate.

Examples of suitable gallium-containing compounds which may be usedinclude gallium acetylacetonate, gallium acetate, GaCl₃, GaBr₃, GaI₃,Ga₂Cl₄ and Ga(OH)₃.

Examples of suitable indium-containing compounds which may be usedinclude indium acetylacetonate, indium acetate, InCl₃, InBr₃, InI₃ andIn(OH)₃.

Examples of suitable tungsten-containing compounds which may be usedinclude W(CO)₆, WCl₄, WCl₆, WBr₅, WI₂, C₉H₁₂W(CO)₃ and any tungstenchloro-, bromo-, or iodo-carbonyl compound.

Preferably, the promoter is present in an effective amount up to thelimit of its solubility in the liquid reaction composition and/or anyliquid process streams recycled to the carbonylation reactor from thecarboxylic acid recovery stage. The promoter is suitably present in theliquid reaction composition at a molar ratio of promoter to rhodium oriridium of 0.1:1 to 20:1, preferably 0.5:1 to 10:1, more preferably 2:1to 10:1. A suitable promoter concentration is less than 8000 ppm, suchas 400 to 7000 ppm.

The liquid reaction composition may also comprise an effective amount ofa stabiliser and/or promoter compound selected from alkali metaliodides, alkaline earth metal iodides, metal complexes capable ofgenerating I—, salts capable of generating I—, and mixtures of two ormore thereof. Examples of suitable alkali metal iodides include lithiumiodide, sodium iodide and potassium iodide, preferably lithium iodide.Suitable alkaline earth metal iodides include calcium iodide. Suitablemetal complexes capable of generating I— include complexes of thelanthanide metals, for example, samarium and gadolinium, cerium, andother metals such as molybdenum, nickel, iron, aluminium and chromium.Salts capable of generating I— include, for example, acetates which arecapable of conversion in-situ to I— typically LiOAc and organic salts,such as quaternary ammonium iodides and phosphonium iodides, which maybe added as such.

Suitably, the amount of stabilising compound used is such that it iseffective in providing an increase in the solubility of the catalystsystem and preferably, does not significantly decrease the carbonylationreaction rate.

Corrosion metals, such as chromium, iron and molybdenum, which may havean adverse affect on the reaction rate, may be minimised by usingsuitable corrosion resistant materials of construction. Corrosion metaland other ionic impurities may be reduced by the use of a suitable ionexchange resin bed to treat the liquid reaction composition, orpreferably a catalyst recycle stream. Such a process is described inU.S. Pat. No. 4,007,130.

The alkyl halide co-catalyst may suitably be a lower, e.g. C₁ to C₄,alkyl halide. Preferably the alkyl halide is an alkyl iodide, such asmethyl iodide. The concentration of alkyl halide co-catalyst in theliquid reaction composition is suitably in the range of from 1 to 30% byweight, for example from 1 to 20% by weight.

In the process of the present invention, a reactant chosen from analcohol and/or a reactive derivative thereof is carbonylated with carbonmonoxide to produce a carboxylic acid and/or the alcohol ester of acarboxylic acid.

A suitable alcohol reactant is any alcohol having from 1 to 20 carbonatoms and at least one hydroxyl group. Preferably the alcohol is amonofunctional aliphatic alcohol, preferably having from 1 to 8 carbonatoms. Most preferably the alcohol is methanol, ethanol and/or propanol.A mixture comprising more than one alcohol may be used. Thecarbonylation product of the alcohol will be a carboxylic acid havingone carbon atom more than the alcohol and/or an ester of the alcohol andthe carboxylic acid product. A particularly preferred reactant ismethanol, the carbonylation product of which is acetic acid and/ormethyl acetate.

Suitable reactive derivatives of an alcohol include esters, halides andethers.

A suitable ester reactant is any ester of an alcohol and a carboxylicacid. Preferably the ester reactant is an ester of a carboxylic acid andan alcohol which alcohol has from 1 to 20 carbon atoms. More preferablythe ester reactant is an ester of a carboxylic acid and a monofunctionalaliphatic alcohol which alcohol has from 1 to 8 carbon atoms. Mostpreferably the ester reactant is an ester of a carboxylic acid andmethanol, ethanol or propanol. Preferably the ester reactant is an esterof an alcohol and the carboxylic acid product. Preferably the esterreactant has up to 20 carbon atoms. A mixture of ester reactants may beused. The carboxylic acid carbonylation product of the ester reactantwill be a carboxylic acid having one carbon atom more than the alcoholcomponent of the ester reactant. A particularly preferred ester reactantis methyl acetate, the carbonylation product of which is acetic acid.

A suitable halide reactant is any hydrocarbyl halide having up to 20carbon atoms. Preferably the halide reactant is an iodide or a bromide.More preferably the halide component of the hydrocarbyl halide reactantis the same halide as that of the alkyl halide co-catalyst. Mostpreferably the hydrocarbyl halide is a hydrocarbyl iodide, mostpreferably methyl iodide, ethyl iodide or propyl iodide. A mixture ofhydrocarbyl halide reactants may be used. The carboxylic acid product ofthe hydrocarbyl halide reactant will be a carboxylic acid having onemore carbon atom than the hydrocarbyl halide reactant. The estercarbonylation product of the hydrocarbyl halide will be the ester of thehydrocarbyl halide and a carboxylic acid having one more carbon atomthan the hydrocarbyl halide.

A suitable ether reactant is any hydrocarbyl ether having up to 20carbon atoms. Preferably the ether reactant is a dialkyl ether, mostpreferably dimethyl ether, diethyl ether or dipropyl ether. A mixture ofethers may be used. The carbonylation products of the ether reactantwill be carboxylic acids having one carbon atom more than each of thehydrocarbyl groups of the ether, and/or ester derivatives thereof. Aparticularly preferred ether reactant is dimethyl ether, the carboxylicacid product of which is acetic acid.

A mixture of alcohol, ester, halide and ether reactants may be used inthe carbonylation process. More than one alcohol, ester, halide and/orether may be used. A particularly preferred reactant is methanol and/ormethyl acetate, the carboxylic acid carbonylation products of which areacetic acid.

The liquid reaction composition may be anhydrous but preferablycomprises a finite concentration of water. By anhydrous as used hereinis meant that the liquid reaction composition is essentially free ofwater, such that the liquid reaction composition comprises less than 0.1wt % water. By finite concentration of water, as used herein, meant thatthe liquid reaction composition comprises at least 0.1 wt % water.Preferably, water may be present at a concentration in the range from0.1 to 30%, for example from 1 to 15%, and more preferably from 1 to10%, by weight based on the total weight of the liquid reactioncomposition.

Water may be added to the liquid reaction composition, where desired, ormay be formed in situ in the carbonylation reaction. For example, in thecarbonylation of methanol, water may be formed by the esterificationreaction between methanol reactant and acetic acid product.

The water may be introduced to the carbonylation reactor together withor separately from the other reactants such as esters, for examplemethyl acetate. Water may be separated from the liquid reactioncomposition withdrawn from the reactor and recycled in controlledamounts to maintain the required concentration in the liquid reactioncomposition.

The carboxylic acid product, for example, acetic acid may be present asa solvent in the liquid reaction composition of the present invention.

The carbon monoxide for use in the present invention (when fedseparately to a hydrogen feed) may be essentially pure or may containinert impurities such as carbon dioxide, methane, nitrogen, noble gases,water and C₁ to C₄ paraffinic hydrocarbons.

The partial pressure of carbon monoxide in the reactor may suitably bein the range from 1 to 70 barg.

Hydrogen may be fed to the reactor separately from the carbon monoxidefeed, but is preferably fed to the reactor as a mixture with carbonmonoxide. Preferably, a mixture of carbon monoxide and hydrogen isobtained from a commercial source such as from the reforming ofhydrocarbons. The commercial reforming of hydrocarbons produces amixture of CO, hydrogen and CO₂, such mixture being generally referredto as syngas. Syngas typically comprises a mol ratio of hydrogen to COin the range 1.5:1 to 5:1.

The mixed hydrogen/carbon monoxide feed may contain at least 1 mol %hydrogen, such as at least 2 mol % hydrogen and, more preferably, atleast 5 mol % hydrogen. The hydrogen to CO mole ratio in the feed ismost preferably between 1:100 and 10:1, such as 1:20 to 5:1.

Where hydrogen is fed to the reactor with CO, the CO consumption in thereactor causes the molar ratio of hydrogen to CO in the reactor to begenerally higher than the molar ratio of hydrogen to CO in the feed tothe reactor. In addition to hydrogen fed to the reaction, hydrogen alsomay be produced in-situ by the water-gas shift reaction. Thus, wherehydrogen is present in the feed to the reactor, and particularly for acarbonylation process operated at high CO conversion, such as a batchprocess, the level of CO in the reactor may become quite low, and themolar ratio of hydrogen to CO in the reactor may get correspondinglyhigh, such as 100:1 or higher. Preferably, however, the hydrogen to COmolar ratio in the reactor is maintained at less than 100:1. Preferably,there is maintained in the carbonylation reactor, hydrogen at ahydrogen:CO mole ratio of at least 1:100. More preferably there ismaintained in the carbonylation reactor, hydrogen at a hydrogen:CO moleratio of at least 1:10, most preferably at least 1:1. The hydrogenpartial pressure in the reactor is preferably greater than 1 bar, mostpreferably greater than 2 bar.

The carbonylation reaction may be carried out at a total pressure in therange from 10 to 100 barg. The temperature may suitably be in the rangefrom 50 to 250° C., typically from 120 to 200° C.

The process may be operated batchwise or continuously, preferablycontinuously.

The carboxylic acid product may be recovered from the liquid reactioncomposition by withdrawing vapour and/or liquid from the carbonylationreactor and recovering carboxylic acid from the withdrawn material.Preferably, carboxylic acid is recovered from the liquid reactioncomposition by continuously withdrawing liquid reaction composition fromthe carbonylation reactor and recovering carboxylic acid from thewithdrawn liquid reaction composition by one or more flash and/orfractional distillation stages in which the acid is separated from theother components of the liquid reaction composition such as rhodium oriridium catalyst, alkyl halide co-catalyst, optional promoter,carboxylic acid ester, unreacted alcohol, water and carboxylic acidsolvent which may be recycled to the reactor.

In a conventional process for the production of a carboxylic acid, apurge is usually taken to keep the hydrogen at low partial pressure inthe reactor (the hydrogen builds up due to impurity levels in the carbonmonoxide feed and hydrogen formed in situ). Since only low levels ofhydrogen can be tolerated, the purge often contains low levels ofhydrogen and significant levels of carbon monoxide, which is disposedof. Since it has now been found that the process of the presentinvention can be operated with higher levels of hydrogen in the reactor,the purge stream will contain higher levels of hydrogen and sosignificantly less carbon monoxide need be purged from the reactor,thereby improving overall CO yield.

A further advantage of the process of the present invention is that highselectivity to the desired liquid products can be achieved in thepresence of hydrogen, allowing carbon monoxide feed streams with highercontents of hydrogen to be employed in the carbonylation process. Thishas significant cost savings. In particular, utilising a carbon monoxidefeed with greater than 1% H₂ allows less expensive, non-cryogenic,methods of syngas separation to be employed, such as membrane separationtechnologies.

The invention will now be illustrated by way of example only and withreference to the following examples:

EXAMPLES

General Reaction Method

Methanol, methyl iodide, RuCl₃.hydrate and dppp(dppp=bis-1,3-diphenylphosphinopropane) were obtained from Aldrich. The(acac)Rh(CO)₂, Xantphos and BINAP were obtained from Strem Chemicals.RuCl₃ was obtained from Johnson Matthey.

Experiments were performed using a 300 ml zirconium autoclave equippedwith a magnetically driven stirrer with a gas dispersion impellersystem, liquid catalyst injection facility and cooling coils. The gassupply to the autoclave was provided from a ballast vessel, feed gasbeing provided to maintain the autoclave at a constant pressure duringreaction.

Comparative Example A

This experiment demonstrates the reaction of methanol with carbonmonoxide in the presence of hydrogen, a rhodium catalyst, dppp and aruthenium promoter during a 2 hour run time. Dppp is a bidentatephosphine ligand. Syngas comprising hydrogen and carbon at a H₂:CO molratio of 2:1 was used (no CO₂ was present in the syngas). 2.031 gram of(dppp)Rh(COMe)I₂ and 2.115 gram of RuCl₃ were suspended in a portion ofthe methanol charge and charged to the autoclave. The reactor was thenpressure tested with nitrogen, vented via a gas sampling system, andflushed with synthesis gas three times. The remaining liquid componentsof the reaction composition (the remaining methanol and methyl iodide)were charged to the autoclave via a liquid addition port. The autoclavewas then pressurised with 5 barg of syngas and slowly vented. Theautoclave was then pressurised with synthesis gas (approximately 20barg) and heated with stirring (1220 r.p.m.) to reaction temperature,140° C. Once stable at temperature (about 15 minutes), the totalpressure was raised to the desired operating pressure by feeding syngasfrom the ballast vessel. The reactor pressure was maintained at aconstant value (±0.5 barg) by feeding gas from the ballast vesselthroughout the experiment. Gas uptake from the ballast vessel wasmeasured using datalogging facilities throughout the course of theexperiment. The reaction temperature was maintained within ±1° C. of thedesired reaction temperature by means of a heating mantle connected to aEurotherm (Trade Mark) control system. After a suitable time, T, (seeTable 1b), the ballast vessel was isolated and the reactor rapidlycooled by use of the cooling coils.

Product distribution data is given in Table 2, product selectivity datais given in Table 3. The predominating liquid products are ethanol andits derivatives (EtOMe and Et₂O) plus its precursor acetaldehyde. Aceticacid and its derivative MeOAc are formed in relatively small amounts.

Comparative Example B

This experiment demonstrates the reaction of methanol with carbonmonoxide in the presence of hydrogen, a rhodium catalyst, dppp and aruthenium promoter during a 30 min run time. Syngas comprising hydrogenand carbon at a H₂:CO mol ratio of 2:1 was used (no CO₂ was present inthe syngas).

In this experiment the phosphine-rhodium complex was generated in situ.1.114 gram of dppp was placed in a portion of the methanol charge (ca.60 g) with 0.658 gram of (acac)Rh(CO)₂ to form a catalyst precursorsuspension. 2.590 gram of RuCl₃.3H₂O was placed in the autoclavetogether with approximately 5 gram of methanol and the autoclave waspressure tested. The MeI co-catalyst was added to the autoclave followedby the catalyst precursor suspension. The remaining methanol was addedand the autoclave was pressurised with syngas (approximately 20 barg).The experiment was then conducted as for Comparative Example A. Reactionconditions are given in Table 1b. Product distribution data is given inTable 2, product selectivity data is given in Table 3. The predominantliquid products are ethanol plus its precursor acetaldehyde. Acetic acidand its derivative MeOAc are formed in relatively small amounts.

Comparative Example C

This experiment demonstrates the reaction of methanol with carbonmonoxide in the presence of hydrogen, a rhodium catalyst, dppp, but inthe absence of a ruthenium promoter, during a 2 hour run time. Syngascomprising hydrogen and carbon at a H₂:CO mol ratio of 2:1 was used (noCO₂ was present in the syngas).

The reaction was performed according to the method of ComparativeExample B using a charge composition and reaction conditions as shown inTables 1a and 1b below. Product distribution data is given in Table 2.Product selectivity data is given in Table 3. In the absence ofruthenium the main liquid product is acetaldehyde. Acetic acid and itsderivative MeOAc are also formed.

Example 1

This example demonstrates the reaction of methanol with carbon monoxidein the presence of hydrogen, a rhodium Xantphos based catalyst and aruthenium promoter. Syngas comprising hydrogen and carbon at a H₂:CO molratio of 2:1 was used (no CO₂ was present in the syngas).

In this experiment the phosphine-rhodium complex was generated in situ.1.571 gram of Xantphos was placed in a portion of the methanol charge(ca. 60 g) with 0.646 gram of (acac)Rh(CO)₂ and 2.084 gram of RuCl₃ toform a catalyst precursor suspension. The MeI co-catalyst was added tothe catalyst injection system along with a small amount of methanol (5gram). The catalyst precursor suspension was added to the autoclave,followed by the remaining methanol and the autoclave was pressurisedwith syngas (approximately 20 barg). The experiment was then conductedas for Comparative Example A, using a charge composition and reactionconditions as given in Tables 1a and 1b below. Product distribution datais given is Table 2. Product selectivity data is given in Table 3.

Example 2

This example demonstrates the reaction of methanol with carbon monoxidein the presence of hydrogen, a rhodium Xantphos based catalyst, and inthe absence of a ruthenium promoter. Syngas comprising hydrogen andcarbon at a H₂:CO mol ratio of 2:1 was used (no CO₂ was present in thesyngas).

The reaction was performed according to the method of ComparativeExample C using a charge composition and reaction conditions as given inTables 1a and 1b below. Product distribution data is given is Table 2.Product selectivity data is given in Table 3.

Examples 3 to 13

Examples 3 to 11 were conducted according to the method of ComparativeExample B using charge compositions and reaction conditions as shown inTables 1a and 1b. Product distribution data is given is Table 2. Productselectivity data is given in Table 3.

TABLE 1a Charge compositions for rhodium catalysed reactions in a 300 mlzirconium batch autoclave. Ligand Complex (acac)Rh(CO)₂ RuCl₃ MeOH MeIExample Ligand (g) (g) (g) (g) (g) (g) A 2.031 0 2.115 80.05 14.50 BDppp 1.114 0.658   2.590(*) 79.35 14.36 C Dppp 1.215 0.637 0    79.7514.58 1 Xantphos 1.571 0.646 2.084 79.48 14.58 2 Xantphos 1.571 0.6510    78.47 14.49 3 BINAP 1.692 0.651 2.032 79.62 14.40 4 oTol-Xantphos0.711 0.267 0.860 79.42 14.87 5 Nixantphos 0.749 0.318 1.079 79.37 7.626 Dpp-Benz 1.215 0.650 2.079 79.99 10.13 7 TRIPHOS 1.468 0.659 2.10280.85 14.46 8 BIPHEP 1.436 0.646 2.114 80.02 14.53 9 TERPHOS 1.742 0.6562.135 79.36 14.46 10 PNP-Phos 1.136 0.606 1.975 81.15 15.07 11 TERPY0.662 0.659 2.112 79.89 14.65 12 BISBI 1.153 0.652 2.109 79.90 14.46 13Dpp-eae 1.519 0.512 1.684 80.26 14.32 (*)(H₂O)₃RuCl₃ used as theruthenium source.

The structures of the ligands of Examples 3-4 and 6-13 are as follows:

TABLE 1b Reaction conditions and gas uptake during the reaction.

Reaction Reaction Reaction temperature pressure Time/ Pressure ExampleT(° C.) P(bar) mins drop (bar) A 140 67 120 61.8 B 140 67 30 13.4(26.8*) C 140 70 120 17.9 1 140 68.7 17 5.8 2 140 68.4 21 7.3 3 140 68.945 0.7 4 140 68 80 10.2 5 140 67 120 11.1 6 140 67 120 14.8 7 140 68 12016.4 8 140 67.7 120 15.4 9 140 68 120 25.1 10 140 67 120 11.6 11 14065.9 103 16.1 12 140 66.8 51 9.0 13 140 66.9 33 10.0 *Experiment indifferent autoclave with larger ballast vessel, recalculated gas uptake26.8 bar may be compared to the other experiments

TABLE 2 Product Distribution. Example MeOH AcOH MeOAc EtOH Et₂O EtOMeMe₂O AcH A 28.6 1.1 4.5 14.2 0.4 3.5 8.2 0.9 B 54.0 0.3 3.7 5.3 0.1 ND7.7 1.9 C 35.1 0.4 2.8 <0.05 0.1 <0.05 10.8 3.1 1 51.7 0.9 14.15 0.1 0.00.8 2.9 0.1 2 50.8 1.0 15.4 0.0 0.0 0.0 4.1 0.1 3 60.2 0.1 4.3 0.1 0.10.7 7.4 0.1 4 40.7 0.8 9.0 1.1 0.1 ND 9.7 0.4 5 48.5 1.1 13.1 1.3 0.1 ND7.7 0.3 6 41.7 1.7 13.4 2.5 0.2 ND 8.4 0.1 7 34.4 2.0 11.2 2.1 0.1 ND10.0 0.7 8 35.9 1.6 10.6 1.9 0.3 ND 8.9 1.0 9 41.6 0.8 7.2 6.0 0.2 ND9.4 0.4 10 44.7 0.5 5.9 3.2 0.1 ND 12.4 0.2 11 32.9 1.6 9.3 3.2 0.1 ND10.8 0.7 12 40.0 2.1 13.6 0.3 0.1 ND 7.4 0.3 13 39.8 1.3 11.9 0.6 0.1 ND7.2 0.7 ND = none detected

TABLE 3 Product selectivity Sel. Sel. AcOH MeOH EtOH and and Sel. Sel.conversion Derivatives Derivatives AcH CH₄ Example %^((a)) %^((b))%^((c)) %^((d)) % A 40.5 66.4 15.7 3.4 14.4 B 16.8 42.7 20.0 15.3 21.9 C38.8 1.2 28.1 42.9 26.9 1 31.1 2.6 35.7 0.5 60.7 2 29.2 0 38.3 0.3 60.93 28.7 17.4 71.3 1.4 9.0 4 29 10.6 54.7 3.7 30.7 5 25 6.5 38.8 1.4 52.76 34 10.9 36.6 0.4 52.0 7 40 9.9 35.3 3.0 51.9 8 34 10.6 34.8 4.6 49.9 934 46.9 38.5 3.0 11.6 10 23 40.3 48.9 2.5 6.7 11 57 15.2 30.6 3.1 50.812 38 1.7 37.6 1.2 59.3 13 36 3.3 35.3 3.1 58.2 ^((a))Methanolconversion was calculated from the recovered methanol in the liquidproduct (Conversion % = 100 * (moles MeOH_(init) − molesMeOH_(recov))/moles MeOH_(init)). Typicalmass balance is of the order of80–90%, the main loss being that of volatile DME on venting theautoclave. For the purpose of calculation DME and the OMe groups in thecompounds MeOEt, MeOAc and Dimethoxyethane are considered as unreactedmethanol. ^((b))The selectivity to ethanol and derivatives was based onthe sum of the selectivity to EtOH and the ethyl groups in, Et₂O, MeOEtand EtOAc in the total liquid products recovered. ^((c))The selectivityto acetic acid and derivatives was based on the sum of the selectivityto acetic acid and the acetate groups in AcOH, MeOAc and EtOAc in thetotal liquid products recovered. ^((d))The selectivity acetaldehyde andderivatives was based on the sum of the selectivity to acetaldehyde andthe ethylidene group in dimethoxyethane in the total liquid productsrecovered. ^((e))The selectivity to methane was based on the amount ofmethane analysed in the autoclave headspace at the end of the reaction.

From an inspection of Tables 2 and 3 it can be clearly be seen that forExamples 1 to 11 using rigid metal-ligand catalysts and for Examples12-13 using catalysts having a bite angle of at least 145° there is asubstantial decrease in ethanol and ethanol derivatives compared to theresults obtained for Comparative Examples A and B. Furthermore the mainliquid carbonylation product is a mixture of acetic acid and methylacetate.

In Examples 3 and 4 it can also be seen that there is a substantialreduction in methane formation for BINAP and o-tol-Xantphos containingcatalysts compared to the Xantphos based catalysts of Examples 1 and 2.

1. A process for the production of a carboxylic acid and/or the alcoholester of a carboxylic acid, which process comprises carbonylating analcohol and/or a reactive derivative of the alcohol which is selectedfrom the group consisting of esters, halides, ethers and mixturesthereof, thereof with carbon monoxide in a liquid reaction compositionin a carbonylation reactor, said liquid reaction composition comprisingsaid alcohol and/or a reactive derivative thereof, a carbonylationcatalyst and an alkyl halide co-catalyst and optionally a finiteconcentration of water, wherein said catalyst comprises at least one ofrhodium or iridium which is coordinated with a polydentate ligandwherein said polydentate ligand has a bite angle of at least 145° orforms a rigid Rh or Ir metal-ligand complex and wherein said polydentateligand comprises at least two coordinating groups which independentlycontain P, N, As or Sb as the coordinating atom of at least two of theco-ordinating groups and wherein in said process there is maintainedhydrogen at a hydrogen:CO mole ratio of at least 1:100 and/or the carbonmonoxide feed to the carbonylation reactor contains at least 1 mol %hydrogen.
 2. A process according to claim 1 wherein the flexibilityrange of the catalyst is less than 40°.
 3. A process according to claim1 wherein the polydentate ligand is a bidentate ligand or a tridentateligand.
 4. A process according to claim 3 wherein the polydentate ligandis a bidentate ligand of which the two co-ordinating groups eachcomprise phosphorous as the co-ordinating atom.
 5. A process accordingto claim 4 wherein the two co-ordinating groups are of formula R¹R²P andR³R⁴P wherein R¹, R², R³ and R⁴ are each independently selected fromunsubstituted or substituted alkenyl groups, alkyl groups and arylgroups.
 6. A process according to claim 5 wherein one or more of thearyl groups are substituted or unsubstituted phenyl groups.
 7. A processaccording to claims 5 or 6 wherein R¹ to R⁴ are each a substituted orunsubstituted phenyl group.
 8. A process according to any one of claims1 to 5 wherein the polydentated ligand is selected from the structuresof formulas 1 to 3 and 1a

wherein P1 and P2 are R¹R²P and R³R⁴P respectively in which R¹, R², R³and R⁴ are each independently selected from unsubstituted or substitutedalkenyl groups, alkyl groups and aryl groups; R⁵ and R⁶ are eachindependently selected from hydrogen, an alkyl group, an aryl group ormay be linked so as to form an aromatic ring.
 9. A process according toclaim 8 wherein at least one of R¹ to R⁴ is a substituted orunsubstituted phenyl group.
 10. A process according to claim 1 or claim2 in which the polydentate ligand is a tridentate ligand.
 11. A processaccording to claim 10 in which the co-ordinating atoms of theco-ordinating groups are in a meridional co-ordination mode with respectto the rhodium or iridium metal centre.
 12. A process according to claim10 in which the co-ordinating atoms of the co-ordinating groups are inan essentially planar configuration with respect to the rhodium oriridium metal centre.
 13. A process according to claim 10 in which thethird co-ordinating group has a co-ordinating atom selected from P, As,Sb, oxygen, nitrogen, sulphur and carbene.
 14. A process according toclaim 13 and wherein two of the co-ordinating groups are as defined inany one of claims 5 to
 7. 15. A process according to claim 10 whereinthe tridentateLligand is of formula L1(R⁷)L3(R⁸)L2 wherein L1 to L3 areeach a co-ordinating group; L1 and L2 each comprising P, N, As or Sb asthe co-ordinating atom; R⁷ and R⁸ are independently selected from anaryl or an alkenyl group or together form a cyclic structure.
 16. Aprocess according to claim 15 wherein R⁷ and R⁸ are independentlyselected from a vinylic and a phenyl group.
 17. A process according toclaim 15 in which the tridentate ligand co-ordinates to the rhodium oriridium metal centre in a bridging conformation such that L1 and L2 aremutually trans with respect to the metal centre.
 18. A process accordingto claim 15 wherein L1 and L2 each comprise phosphorous as theco-ordinating atom and L3 has a co-ordinating atom selected from oxygen,nitrogen and sulphur.
 19. A process according to claim 18 wherein theco-ordinating atom of L3 is oxygen.
 20. A process according to claim 18or claim 19 wherein L1 and L2 are represented by R¹R²P and R³R⁴Prespectively in which R¹, R², R³ and R⁴ are each independently selectedfrom unsubstituted or substituted alkenyl groups, alkyl groups and arylgroups.
 21. A process according to claim 20 wherein each of R¹ to R⁴ isa substituted or unsubstituted phenyl group.
 22. A process according toclaim 21 wherein each of R¹ to R⁴ is an unsubstituted phenyl group. 23.A process according to claim 15 wherein L1 , L2 and L3 are each anitrogen atom.
 24. A process according to claim 10 wherein thetridentate ligand is selected from the group consisting of xantphos,thixantphos, sixantphos, homoxantphos, phosxantphos, isopropxantphos,nixantphos, benzoxantphos, DPEphos, DBFphos and alkyl and arylderivatives thereof.
 25. A process according to claim 24 in which theoxygen atom of the tridentate ligands is substituted by nitrogen orsulphur.
 26. A process according to claim 25 wherein at least one of thephosphorous co-ordinating atoms is substituted by an arsenic or antimonyatom.
 27. A process according to claim 23 in which the tridentate ligandis a substituted or unsubstituted terpyridine.
 28. A process accordingto claim 1 or claim 2 wherein the catalyst comprises rhodium.
 29. Aprocess according to claim 1 or claim 2 wherein the catalyst is added tothe liquid reaction composition as a performed metal-polydentate ligandcomplex or is formed in-situ in the liquid reaction composition.
 30. Aprocess according to claim 1 or claim 2 wherein the mol ratio of iridiumor rhodium metal to polydentate ligand is in the range 1:1 to 1:2.
 31. Aprocess according to claim 1 or claim 2 wherein the liquid reactioncomposition further comprises a catalyst promoter.
 32. A processaccording to claim 31 wherein the promoter is selected from the groupconsisting of ruthenium, osmium, rhenium, cadmium, mercury, zinc,gallium, indium and tungsten.
 33. A process according to claim 1 orclaim 2 in which the liquid reaction composition also comprises aneffective amount of a compound selected from the group consisting ofalkali metal iodides, alkaline earth metal iodides, metal complexescapable of generating I—, salts capable of generating I— and mixturesthereof.
 34. A process according to claim 1 or claim 3 wherein the alkylhalide co-catalyst is a C₁ to C₄ alkyl halide.
 35. A process accordingto claim 1 or claim 2 wherein the alcohol is a C₁ to C₈ aliphaticalcohol.
 36. A process according to claim 35 wherein the alcohol isselected from methanol, ethanol, the propanols and mixtures thereof. 37.A process according to claim 1 or claim 2 wherein the liquid reactioncomposition comprises water in a concentration in the range 0.1 to 30 wt%.
 38. A process according to claim 37 wherein the water concentrationis in the range 1 to 10 wt %.
 39. A process according to claim 1 inwhich carbon monoxide and hydrogen are fed separately or as a mixture tothe reactor.
 40. A process according to claim 39 wherein the carbonmonoxide and hydrogen are fed to the reactor as a mixture.
 41. A processaccording to claim 40 wherein the mixture of hydrogen and carbonmonoxide is obtained from the reforming of hydrocarbons.
 42. A processaccording to claim 41 wherein the ratio of hydrogen to carbon monoxideis in the range 1.5: to 5:1.
 43. A process according to claim 40 orclaim 41 wherein the mixture comprises at least 2 mol % hydrogen.
 44. Aprocess according to claim 40 wherein the mol ratio of hydrogen tocarbon monoxide is in the range 1:100 to 10:1.
 45. A process accordingto claim 1 or claim 2 wherein there is maintained in the processhydrogen at a hydrogen to carbon monoxide mol ratio of at least 1:10.46. A process according to claim 45 wherein the hydrogen:carbon monoxidemol ratio is at least 1:1.
 47. A process according to claim 1 or claim 2wherein the hydrogen partial pressure is greater than 1 bar.
 48. Aprocess according to claim 1 wherein the bite angle is at least 150°.49. A process according to claim 1 to claim 2 wherein the product of thecarbonylation process is selected from acetic acid, methyl acetate andmixtures thereof.
 50. A process according to claim 2 wherein thepolydentate ligand is a bidentate ligand or a tridentate ligand.